1. Field of the Invention
The present description relates generally to DC-DC voltage converters.
2. Description of the Related Art
Switched-mode DC-DC voltage converters provide superior voltage conversion efficiency because they regulate an output voltage with transistor switches that are either on or off so that they never operate in the linear region in which both current and voltage are nonzero. Because at least one of transistor current and voltage is therefore always close to zero, dissipation is greatly reduced and the converter efficiency is quite high.
These converters typically include a voltage feedback loop in which the difference between the output voltage and a reference voltage forms an error voltage Verr. The error voltage can be compared to a fixed voltage ramp to provide a pulse-width modulation signal that determines the duty cycles of a converter's first and second transistors. It has been found that this voltage feedback loop can be augmented with a current feedback loop which replaces the fixed voltage ramp with a ramp whose amplitude is proportional to the converter's input current. The current feedback loop enables the voltage converter to respond more quickly to changes in the input voltage.
These voltage converters are especially efficient when delivering medium-to-high load currents. As the load current drops to lower levels, however, switching losses in the converter's transistors become high relative to the output power and converter efficiency suffers. Various operational modes have been introduced to improve this low-current efficiency but they are generally complex and are typically structured to control only one parameter, e.g., feedback voltage or peak inductor current, which limits their effectiveness.